Fuel injection nozzles for Diesel engines have to be capable of injecting fuel in substantially varying quantity, and under substantially varying pressure. Upon starting of a Diesel engine, and also during idling, or under low load conditions, a comparatively long period of time is available for carrying out the injection of fuel; as the speed increases, the fuel injection time decreases. Yet, at low speed and low load, only little fuel is needed. Various parameters of the injection fuel relationships and functions, such as cross section of the injection nozzle opening, stroke of the injection valve needle, closing force of the needle, and hence opening force, and injection pressure, are usually so arranged that the injection valve can supply the engine with required fuel at full-load operation and at maximum speed, thus obtaining maximum power output. Upon low-speed, low-power operation of the engine, and particularly upon starting, upon idling, and upon low load, only a small quantity of fuel, per unit time, will flow through a comparatively large cross-sectional opening of the nozzle. It has already been proposed to provide an injection valve which, as the needle stroke changes, provides fluid flow communication to sequentially positioned injection openings. To inject small quantities of fuel, the valve is controlled to have only a small stroke, so that, effectively, a smaller injection opening is commanded to be exposed than the injection opening available for larger quantities. It is quite possible, however, that the needle valve will open fully due to the initial pressure pulse, so that the entire and relatively small quantity of fuel is initially and immediately injected. The result is that the fuel is well atomized, considering the small quantity, but that the engine noise is considerable. Further, the needle valve may vibrate due to oscillations deriving from the counter spring or pressure waves which will occur in the supply line, resulting in noisy operation of the engine.
It has also been proposed to control movement of the needle valve upon starting, and otherwise upon low-speed operation of the engine, by providing a high-low control pressure stage which is so arranged that a force acts on the valve needle which increases as the opening stroke increases. Various solutions to so control the needle are known, for example the addition of a second closing spring, which becomes effective as the stroke of the needle increases. Constructing needle valves in accordance with such proposals increases the expense of construction and further the space required for building the injection valve.